Ultrahigh pressure water is converted into a high velocity waterjet within a cutting head of an entrainment abrasive waterjet apparatus. The waterjet traverses a chamber within the cutting head that has a passaged connection through which abrasive particles in a carrier fluid enter the chamber. The waterjet passes out the chamber entering a contracting inlet and bore of a focus tube entraining abrasive particles and carrier fluid into the focus tube bore. In the focus tube bore momentum is exchanged between the waterjet and abrasive particles to generate an abrasive cutting jet at a focus tube outlet.
For brevity an abrasive cutting jet is hereafter referred to as a cutting jet. Focus tubes are also known in the art as mixing tubes, abrasive waterjet nozzles and nozzles.
Cutting jets with diameters above 200 microns require abrasive particles with mean diameters greater than 40 microns or so to cut effectively. Such particles are free flowing, easy to meter and to transport dynamically by airflow in tubing to a cutting head. Generating cutting jets with diameters less than 200 microns requires abrasive particles with diameters less than 40 microns or so, with particle diameters dropping to 10 microns or so for a 50 micron diameter cutting jet. As abrasive particle diameters diminish below 40 microns inter-particle forces and friction between particles increase rapidly so that particles flow less readily and particle clumping becomes a serious problem because of rapid moisture absorption by fine abrasive exposed to the environment. When carried in airflow particles less than 40 microns or so in diameter tend to attach to tube walls and cutting head passage walls as a result of electrostatic forces. Particles attached to cutting head passage walls can be wetted, particularly during the starting and stopping of water flow and this can lead to clumping of particles followed by blockage. These factors make it difficult or impossible to meter and feed abrasive particles less than 40 microns mean diameter or so from a hopper to a cutting head by dynamically suspending abrasive particles in airflow.
When particle diameters are such that they cannot be satisfactorily metered and carried dynamically to a cutting head in airflow it is necessary to change the particle carrier fluid to water or another liquid. The transport method of this patent application involves temporarily suspending abrasive particles in sufficient water or other liquid to flow to a cutting head. Since water is the preferred liquid this is referred to thought out the text. Abrasive particles that are temporarily suspended in water are referred to as abrasive suspension thought out the text. The term abrasive in the text is taken to mean abrasive particles.
Cutting heads that entrain abrasive suspension have been known in the art for over 30 years but have not been exploited commercially for precision machining. Reasons for this include poor cutting performance, relative to cutting heads that entrain abrasive in airflow, and complex, unreliable and difficult to operate abrasive suspension feed systems.
Many applications in micromachining would benefit from the unique cutting capabilities of abrasive waterjets. The poor cutting performance of cutting heads that entrain abrasive suspensions has recently been addressed by the inventor in patent applications EP 2 097 223 B1, and WO2011/070154 A1. In particular the cutting heads of said patent applications can be used to generate cutting jets with diameters down to 50 microns or so. To exploit the cutting heads of said patent applications requires the development of abrasive suspension feed systems that are effective, easy to use and reliable.
The processes involved in feeding temporarily suspended abrasive in water to a cutting head are fundamentally different and considerably more complex than the processes involved in carrying abrasive to a cutting head dynamically suspended in airflow. When abrasive is temporarily suspended in water rheological processes are involved that are both time and shear dependent and these processes affect metering, flow behaviour and phase separation by gravity and water migration. Importantly a number of cutting head abrasive feed parameters that are independent parameters when abrasive flows to a cutting head dynamically suspended in airflow become dependent parameters when a cutting head entrains abrasive suspension. The change from independent to dependent cutting head parameters greatly increases the level of control and automation needed to operate abrasive waterjet apparatus as part of a machine tool compared to abrasive waterjet apparatus that feed abrasive to a cutting head dynamically suspended in air
Prior art, such as described in U.S. Pat. No. 4,872,293, demonstrates that fine abrasive suspensions can be fed to cutting heads but do not teach how to design abrasive feed systems that make machining with abrasive suspensions commercially practical. Prior art does not address a number of aspects of the design and operation of abrasive suspension feed systems that must be satisfied when abrasive waterjet apparatus is part of a machine tool. These aspects include avoiding particle jamming followed by flow passage blockage, avoiding settling of abrasive suspensions during machining cycles, control over abrasive concentration in abrasive cutting jets and the automation and integration of an abrasive suspension feed system with a machine tool. All of these aspects are addressed by the present invention.
The flow of solid/liquid mixtures with a high solids concentration are particularly prone to particle jamming that leads on to blockage formation in conduits and within flow system components. Once jamming occurs a blockage develops and grows as liquid percolates through the blockage and additional particles are deposited. Increasing pressure to clear a blockage can consolidate the blockage. This means a system usually has to be dismantled to physically remove a blockage. Minimising problems caused by blockages in solids/liquid flow systems plays an important role in the design and operation of solid/liquid flow systems. Prior art related to abrasive suspension feed systems for cutting heads of abrasive waterjet apparatus is quiet about jamming problems.
The smaller the particle size the lower the risk of blockage formation. Other parameters being equal the greater the particle diameter the higher the cutting speed. To achieve efficient cutting it is necessary to cut with particles having mean diameters that approach those that could lead to a blockage occurring in the bore of a cutting head focus tube. Particle diameters are desirably 20% or so of the diameter of a focus tube bore. Although not explicitly stated, prior art abrasive suspension feed systems, such as described in U.S. Pat. No. 4,872,293, did not optimise particle sizes relative to focus tube diameters and operated with abrasive particles too small for efficient cutting. Such systems are much less prone to blockages than systems operating with particle sizes optimised for efficient cutting.
Blockages are most likely at flow restrictions, regions of settled abrasive in tubing and flow passages and when de-watering occurs due to water wicking or migration. The accepted practice in the design of solid/liquid flow systems is to avoid steeply sloping conduits in order to minimise movement of solids when flow is stopped. In steeply sloping and vertical sections of small bore conduits, such as plastic tubing used in connections to abrasive waterjet cutting heads, abrasive particles tend to settle to form a series of plugs with essentially clear water in between. Because particles only settle over a distance of a few tube diameters blockages form quickly in steeply sloping tubing. Prior art is silent about the problems of abrasive settling in steeply sloping small bore tubing. Steeply sloping small bore conduits are unavoidable in feeding abrasive suspension to a cutting head of an abrasive waterjet apparatus so the abrasive suspension feed systems described in this patent application are designed and their operation automated to prevent problems caused by abrasive settling in steeply sloping tubing.